1. Field of the Invention
The present invention relates to a dynamic image coding and decoding method and a related apparatus for compressing and expanding digitized dynamic image signals, and more particularly to a method and an apparatus robust against an error occurring during a transmission.
2. Prior Art
First, a conventional image coding and decoding method will be explained schematically. A motion compensate/predict coding method and a variable-length coding method are conventionally known as fundamental techniques for coding and decoding dynamic images. A copy information method is a transmitting technique capable of increasing the robustness against a transmission error occurring on a transmission line. FIG. 6 shows a quarter common input format (abbreviated QCIF) which is used worldwide as a standard format for a dynamic image. The quarter common input format shown in FIG. 6 represents a brightness signal of lateral 176 pixels by vertical 144 pixels. In general, an input image is split into a plurality of blocks, each comprising 16 by 16 pixels and serving as units for a coding. Each split block is referred to as macro block. Thus, one frame is constituted by 99 macro blocks.
Next, the motion compensate/predict coding method will be explained. In each macro block, a present macro block is compared with a corresponding image reproduced one frame before to calculate a shift amount of the present macro block in both horizontal and vertical directions relative to the one frame ahead reproduced image. The shift amount, thus obtained, is referred to as a motion vector.
Then, a shift operation is performed independently in each macro block to produce a predicted image. More specifically, the image reproduced one frame before is shifted in the direction parallel to the motion vector of the present macro block by an amount equivalent to the length of this motion vector. Then, the present macro block is replaced by the resultant image, thereby producing the predicted image.
A difference between an actual input image and a corresponding predicted image is calculated in each macro block. The difference thus obtained is referred to as a predicted error. This predicted error is transformed into a frequency region. A discrete cosine transformation is used for this transformation. The discrete cosine transformation factor (hereinafter, referred to as cosine factor) is then quantized and transmitted.
The quantized value is processed through a reverse quantization and a reverse discrete cosine transformation and reconstructed as a predicted error. The predicted error is added to the predicted image. The reproduced image, thus obtained, is used for calculating a motion vector of the next input image as well as producing the predicted image of the next input image.
Next, the variable-length coding operation will be explained. The variable-length coding operation is performed for processing the calculated motion vector value and the quantized cosine factor. Hereinafter, a variable-length coding operation performed on the motion vector will be explained. A occurrence frequency of each motion vector is obtained statistically. A motion vector having a higher occurrence frequency is expressed by a short bit. A motion vector having a lower occurrence frequency is expressed by a large bit. By doing so, it becomes possible as a whole to suppress the total volume of codes to be generated. Similarly, the above-described cosine factors and other control information are processed based on the statistical occurrence frequency, so that the total volume of the generated codes can be suppressed.
Next, the copy information transmission will be explained. The copy information transmission is based on a repetitive transmission of particular information. More specifically, important information required for expressing one frame image is transmitted plural times to prevent the image quality from being deteriorated due to an error generated during a transmission. When the image information transmitted first is not reproduced at a decoder side due to a transmission error, the failed image information is replaced by the corresponding copy information transmitted as important information in the succeeding transmission. Thus, it becomes possible to effectively suppress the deterioration of the image quality.